This invention relates to an ignition system for an internal combustion engine, and more particularly to an ignition system of the current interruption type using a magneto as a power supply.
For an internal combustion engine is widely used an ignition system of the current interruption type which is adapted to interrupt a primary current flowing through an ignition coil device arranged in a magneto driven by the internal combustion engine to generate a high voltage for ignition. Such an ignition system of the current interruption type is disclosed in, for example, U.S. Pat. No. 4,163,437, U.S. Pat. No. 4,175,509 and U.S. Pat. No. 4,173,963.
A magneto used in such an ignition system generally includes a three-pole magnetic rotor including a first rotor magnetic pole comprising a magnetic pole defined on the side of the outer periphery of a magnet centrally positioned in a recess provided on the outer periphery of a rotor body made of a magnetic material, and a second rotor magnetic pole and a third rotor magnetic pole respectively comprising magnetic poles defined on the peripheral portions of the rotor body defined by dividing the outer periphery of the rotor body into two sections through the recess. Also, the magneto includes an ignition coil device including a coil wound on a stator core provided at both ends thereof with a pair of stator magnetic poles which are defined so as to be opposite to the magnetic poles of the magnetic rotor. An example of the three-pole magnetic rotor constructed as described above is disclosed in U.S. Pat. No. 4,566,425.
The ignition coil device is provided on the primary side thereof with a transistor switch circuit connected in parallel to a primary coil of the ignition coil device and an interruption control circuit for detecting a voltage across the transistor switch circuit or primary coil and interrupting the transistor switch circuit when the detected voltage reaches a predetermined level. The output of the ignition coil device on the side of its secondary coil is applied to an ignition plug mounted on a cylinder of the engine.
The magnetic rotor is generally mounted on an output shaft of the engine. Rotation of the engine causes an AC voltage to be induced across the primary coil of the ignition coil device. When a half wave voltage of one polarity is induced across the primary coil, the transistor switch circuit is supplied with a base current, resulting in being turned on, leading to flowing of a primary current from the primary coil through the transistor switch circuit. When the primary current reaches a predetermined magnitude, the voltage across the transistor switch circuit or primary coil reaches a predetermined level, so that an interruption control circuit renders the transistor switch circuit turned off or non-conductive to interrupt the primary current, leading to induction of a relatively high voltage across the primary coil. The ignition coil device increases the voltage thus induced to cause a high voltage for ignition to be induced across the secondary coil of the ignition coil device. The high voltage results in spark occurring at the ignition plug, leading to ignition of the engine.
In the conventional ignition system constructed as described above, an increase in engine speed increases the voltage induced across the primary coil of the ignition coil device, so that the voltage across each of the primary coil and transistor switch circuit is increased. This causes an ignition position at which the voltage across the transistor switch circuit or primary coil reaches a predetermined level to be advanced, leading to advance of the ignition position.
In the conventional ignition system of the current interruption type described above, an angular interval from starting of rising-up of the primary current to ascending of the current to a peak value is a theoretical maximum advance angle width. However, an actual advance angle width is considerably small as compared with the maximum advance angle width. An increase in advance angle width requires to increase the angular interval from starting of rising-up of the primary current to ascending of the current to a peak value. Unfortunately, the conventional ignition system using the three-pole magneto fails to increase the angular interval, resulting in failing to increase the advance angle width.
A failure in increase of the advance angle width possibly causes the ignition operation carried out when the engine is started or when the engine is driven at a low speed to be unstable, because an ignition position at which the engine driven at a low speed is ignited is considerably advanced where an ignition position at which an ignition of the engine driven at a high speed is carried out is set at a position advanced by a predetermined angle from the top dead center of the engine. In particular, when the engine is started by means of a hand or foot starter, the starter is pushed back when the engine is driven at a low speed, resulting in a driver being possibly damaged.